1. Field of Invention
The present invention relates to the field of voltage regulators and more specifically to the field of methods and circuits for the control of current levels during output transistor saturation in vertical structure voltage regulators.
2. Description of Related Art
Typical integrated circuit (IC) voltage regulators employ lateral PNP structures. A schematic diagram of one such voltage regulator 10 is shown in FIG. 1. One desireable operational characteristic of a voltage regulator is the prevention of surges in the quiescent current drawn by a voltage regulator from a voltage source when the load across the voltage regulator output is non-existent or very light and the potential of the voltage source approaches the desired or set value of the voltage regulator output. In a case where the voltage source is a battery, such surges significantly accelerate the discharge of the battery.
Referring now to FIG. 1, in operation, an output transistor 12 through a first collector 14 powers an external load (not shown) connected between an output port 16 and common. When the output transistor 12 saturates, a transistor 18 conducts thereby adding sufficient current to the bias level of a transistor 32 to in turn retard the conduction level of a transistor 22. Transistor 18 is of lateral construction and embedded within the structure of output transistor 12, thus facilitating accurate sensing of the onset of saturation. A resistor 34 limits the level of clamping provided by transistor 32. A resistor 36 sets the bias at the base of transistor 32.
Retarding the conduction level of transistor 22 lowers, through a transistor 24, the current through the base of the output transistor 12, thereby regulating the depth of saturation of output transistor 12.
A capacitor 26 provides compensation for a feedback loop which is further described herein. In addition, a second collector 28 of the transistor 12 provides an alternate feedback loop which regulates the depth of saturation of the output transistor 12 through a transistor 30 and a transistor 20.
A voltage divider consisting of a resistor 38 and a resistor 40 provide feedback from the voltage output port 16. This feedback is connected to an inverting input 42 of a control amplifier 44. A non-inverting input 46 of the control amplifier 44 is connected to a voltage reference 48, the potential of which is used to set the desired potential at the voltage output port 16. An output 50 of the control amplifier 44 controls, in part, the base current of the transistor 22, which in turn through the transistor 24 and a resistor 52 further sets the base current of the output transistor 12. A current source 54 operates to bleed off leakage currents which might cause the output transistor 12 to conduct when transistor 22 is not conductive.
Because vertical construction does not provide for multiple collectors such as those utilized in the lateral voltage regulator 10, it would be desireable to provide a method and circuit which provides good control of the rise in operating current of a saturated output transistor in a vertical construction IC voltage regulator.